Glass substrate for magnetic disk and magnetic disk

ABSTRACT

A magnetic-disk glass substrate has a circular hole at a center, and includes a pair of main surfaces and a side wall surface orthogonal to the main surfaces. A roundness of the circular hole is 1.5 μm or less. A difference between a maximum value and a minimum value of radii of three inscribed circles that are respectively derived from outlines in the circumferential direction at three positions spaced apart by 200 μm in a substrate thickness direction on the side wall surface of the circular hole is 3.5 μm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/767,515,filed on Aug. 12, 2015, which is a U.S. National stage application ofInternational Patent Application No. PCT/JP2014/054384, filed on Feb.24, 2014, which, in turn, claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2013-033719, filed in Japan on Feb. 22,2013, the entire contents of which are hereby incorporated herein byreference.

BACKGROUND

Field of the Invention

The present invention relates to a magnetic-disk glass substrate and amagnetic disk.

Background Information

Nowadays, personal computers, digital versatile disc (DVD) recorders,and the like have a built-in hard disk drive (HDD) for data recording.In particular, in a hard disk drive that is used in a device premised onportability such as a notebook-type personal computer, a magnetic diskin which a magnetic layer is provided on a glass substrate is used, andmagnetic recording information is recorded on or read from the magneticlayer with a magnetic head that flies slightly above the surface of themagnetic disk. A glass substrate is unlikely to be plastically deformedcompared with a metal substrate (aluminum substrate) or the like, andthus is preferably used as the substrate of this magnetic disk.

Moreover, the density of magnetic recording has been increased to meetthe demand for an increase in the storage capacity of hard disk drives.For example, the magnetic recording information area has been madesmaller using a perpendicular magnetic recording system that causes thedirection of magnetization in the magnetic layer to be perpendicular tothe surface of the substrate. This makes it possible to increase thestorage capacity per disk substrate. In such a disk substrate, it ispreferable that the substrate surface is made as flat as possible andthe direction in which magnetic particles grow is arranged in thevertical direction such that the direction of magnetization in themagnetic layer faces in a substantially perpendicular direction relativeto the substrate surface.

Also, in order to further increase the storage capacity, by using amagnetic head equipped with a dynamic flying height (DFH) mechanism tomake the flying height of the magnetic head from the magnetic recordingsurface extremely short, the magnetic spacing between the recording andreproducing element of the magnetic head and the magnetic recordinglayer of the magnetic disk is reduced, thus further improving theaccuracy of the recording and reproducing of information (improving theS/N ratio). Also in this case, it is required to make the surfaceunevenness of a magnetic-disk substrate as small as possible in orderfor the magnetic head to stably read/write magnetic recordinginformation over a long period of time.

Servo information that is to be used to position the magnetic head at adata track is recorded on the magnetic disk. It is conventionally knownthat when the roundness of an edge surface of the magnetic disk on theouter circumferential side (also referred to as “outer circumferentialedge surface” hereinafter) is reduced, the magnetic head flies stably,and thus the servo information is favorably read, and the magnetic headstably reads/writes information. For example, the technique described inJP 2008-217918A discloses a magnetic-disk glass substrate in which theroundness of the outer circumferential edge surface is 4 μm or less.With this glass substrate, the durability against load/unload (LUL)testing is improved by reducing the roundness of the outercircumferential edge surface.

SUMMARY

Incidentally, in recent years, a magnetic disk provided with anextremely high track recording density of 500 kTPI or more has becomepractically viable due to the progress of elemental technology such asenergy-assisted magnetic recording technology and shingle writingtechnology. However, when the magnetic disk provided with such anextremely high track recording density was incorporated into a spindleand a HDD was assembled, it was found that there were cases in whicheven when a magnetic disk having a sufficiently reduced (small)roundness of a circular hole at the center (appropriately referred to as“inner hole” hereinafter) was used (magnetic disk in which the roundnessof the inner hole is 1.5 μm or less, for example), a level of fluttering(a phenomenon where the magnetic disk vibrates (rattles) due to therotation of the magnetic disk) increased during high speed rotation ofthe magnetic disk.

In view of this, an object of the present invention is to provide amagnetic-disk glass substrate and a magnetic disk that are capable ofreducing fluttering of the magnetic disk during high speed rotation.

Regarding the above-described issues, the inventors studied variation inthe thickness of a magnetic disk having a high level of fluttering in aHDD and the roughness and microwaviness of a main surface, but noparticular anomalies could be confirmed. The inventors assumed that thereason why the level of fluttering increased was as follows.

In particular, in the magnetic disk using energy-assisted magneticrecording technology, the particle size of magnetic particles is reducedin order to perform high density recording, and in order to suppressdeterioration in magnetic characteristics due to thermal fluctuationresulting from a reduction in the particle size, so-called high Kumagnetic material (such as Fe-Pt-based or Co-Pt-based magnetic materialhaving a high magnetic anisotropy energy) is used. This high Ku magneticmaterial needs to obtain a specific crystal orientation state in orderto achieve a high Ku, and therefore, it is necessary to perform heatprocessing at a high temperature when or after film formation at a hightemperature. In order to form a magnetic recording layer made of thesehigh Ku magnetic materials, a glass substrate needs to have a high heatresistance that can withstand the above-described high temperatureprocessing, or in other words, the glass substrate needs to have a highglass transition temperature (600 to 700° C. or more, for example).There are cases where in order to have a high heat resistance, a glassmaterial having a smaller coefficient of thermal expansion (CTE) thanthat of a conventional glass material is used in a magnetic-disk glasssubstrate using energy-assisted magnetic recording technology.Stretching of the glass substrate during the heat processing can besuppressed by reducing the coefficient of thermal expansion, and thus arisk of deformation or cracking of the disk by a holding member thatholds the substrate during the heat processing, dropping of the diskfrom the holding member, or the like can be reduced.

However, although the coefficient of thermal expansion of the glassmaterial for a conventional magnetic-disk glass substrate is relativelylarge value (from 90×10⁻⁷ to 100×10⁻⁷ [K⁻¹], for example) in order toapproximate to the coefficient of thermal expansion of the spindlematerial, in the case where a magnetic-disk glass substrate is producedusing a glass material having a smaller coefficient of thermal expansionthan that of a conventional glass material, the difference incoefficient of thermal expansion between the materials of themagnetic-disk glass substrate and the spindle is greater than that whenthe conventional glass material is used. It is thought that if thedifference between both coefficients of thermal expansion is great, inthe case where, after a HDD is assembled, the HDD is placed in a hightemperature atmosphere through performing a heat cycle test or the likeon the HDD, the spindle greatly expands relative to the glass substrate,and locally abuts strongly against the inner hole of the magnetic disk,as a result of which the magnetic disk is slightly distorted. In otherwords, it is thought that even in the case where the inner hole of themagnetic disk is provided with sufficiently favorable roundness byconventional standards, if the precision of the three-dimensional shapeof the inner hole is not high, stress is applied due to the spindlelocally abutting strongly against the inner hole of the magnetic disk,and thus the magnetic disk is slightly distorted (warped).

It should be noted that the roundness of the inner hole has beenconventionally measured by inserting a plate-shaped probe that is longerthan the thickness of the glass substrate into the inner hole in thevertical direction with respect to the main surface of the glasssubstrate and scanning the inner hole in the circumferential direction.At this time, the probe comes into contact with the substrate at aposition in the substrate thickness direction that projects mostcentrally. Accordingly, the shape projecting furthest toward the centerof the substrate is reflected in the outline of the inner hole thatserves as a basis of the roundness measurement, independently of theshape of the inner hole in the substrate thickness direction. Therefore,with the conventional method for measuring roundness, the roundnesscould not serve as an index for evaluating the three-dimensional shapeof the side wall surface of the inner hole in the substrate thicknessdirection. In other words, in the case where the inner hole of themagnetic disk was provided with sufficiently favorable roundness basedon the conventional method for measuring the roundness, there may becases where the precision of the three-dimensional shape of the innerhole is not high.

If the magnetic disk is slightly distorted as described above, the levelof fluttering increases when the magnetic disk is rotated at high speeddue to this slight distortion. It is thought that an increase in thelevel of fluttering leads to problems such as the accuracy ofpositioning the magnetic head at a data track of the HDD beingdeteriorated. In particular, with a high TPI HDD, in order to suppressthe eccentricity of the magnetic disk during rotation that adverselyinfluences the accuracy in positioning the magnetic head at a data trackof the HDD, play between the diameter of the spindle and the innerdiameter of the magnetic disk is thought to have become extremely smallat 20 μm or less, and thus it is conceivable that this small play amountfacilitates the spindle coming into local contact with the inner hole ofthe magnetic disk due to the difference in CTE described above. Also,the problem of fluttering resulting from the distortion of the substratedescribed above is more marked in a HDD in which the substrate rotatesat a high speed of 10000 rpm or more.

As a result of further intensive studies based on the above-describedassumption, the inventors found that the degree of fluttering describedabove is associated with the precision of the three-dimensional shape ofthe inner hole of the magnetic disk. In other words, it is conceivablethat even in the case where the roundness of the inner hole of themagnetic disk is sufficiently increased, if the precision of thethree-dimensional shape is not high, the spindle will come into localcontact strongly with the side wall surface of the magnetic disk on theinner hole side and the magnetic disk is likely to be distorted, andthus the level of fluttering increases. On the other hand, it isconceivable that in the case where not only a favorable roundness of theinner hole of the magnetic disk but also a favorable precision of thethree-dimensional shape are achieved, the spindle comes into contact(that is, comes into surface contact) with the entire side wall surfaceof the magnetic disk on the inner hole side, and thus the magnetic diskis unlikely to be distorted, and the level of fluttering is unlikely tobecome significant.

From the point of view described above, a first aspect of the presentinvention is a magnetic-disk glass substrate having a circular hole at acenter, and including a pair of main surfaces and a side wall surfaceorthogonal to the main surfaces, a roundness of the circular hole being1.5 μm or less, and a difference between a maximum value and a minimumvalue of radii of three inscribed circles that are respectively derivedfrom outlines in a circumferential direction at three positions spacedapart by 200 μm in a substrate thickness direction on the side wallsurface of the circular hole being 3.5 μm or less.

A second aspect of the present invention is a magnetic-disk glasssubstrate having a circular hole at a center, and including a pair ofmain surfaces and a side wall surface orthogonal to the main surfaces, asubstrate thickness being 0.635 mm or less, a roundness of the circularhole being 1.5 μm or less, and a difference between a maximum value anda minimum value of radii of three inscribed circles that arerespectively derived from outlines in a circumferential direction atthree positions spaced apart by 100 μm in a substrate thicknessdirection on the side wall surface of the circular hole being 3.5 μm orless.

In the above-described magnetic-disk glass substrate, it is preferablethat a surface roughness Rz of the side wall surface of the circularhole is 0.2 μm or less. In the above-described magnetic-disk glasssubstrate, it is preferable that an average coefficient of thermalexpansion from 100° C. to 300° C. is 60×10⁻⁷ [K⁻¹] or less.

In the above-described magnetic-disk glass substrate, with regard to thesurface roughness of the side wall surface of the circular hole, in acase where a maximum height in the substrate thickness direction isRz(t) and a maximum height in the circumferential direction is Rz(c), itis preferable that Rz(t)/Rz(c) is 1.2 or less. In the above-describedmagnetic-disk glass substrate, when a measurement point is providedevery 30 degrees in the circumferential direction referenced on thecenter of the glass substrate, and a radius of curvature of a shape of aportion between the side wall surface and a chamfered surface of thecircular hole at the measurement point is derived, it is preferable thata difference in the radius of curvature between adjacent measurementpoints is 0.01 mm or less.

A third aspect of the present invention is a magnetic disk in which amagnetic layer is formed on a main surface of the above-describedmagnetic-disk glass substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing an external shape of a magnetic-disk glasssubstrate of the present embodiment;

FIG. 1B is an enlarged cross-sectional view of an inner circumferentialside edge portion of the magnetic-disk glass substrate of the presentembodiment;

FIG. 2A is a diagram illustrating a method for measuring outlines of aside wall surface of the magnetic-disk glass substrate of the presentembodiment;

FIG. 2B is a diagram illustrating a method for measuring outlines of aside wall surface of the magnetic-disk glass substrate of the presentembodiment;

FIG. 3 is a diagram illustrating a method for calculating a shapeevaluation value of an inner hole based on the outlines of the side wallsurface of the magnetic-disk glass substrate of the present embodiment;

FIG. 4 is an enlarged view of a portion of an inner circumferentialcross-section of FIG. 1; and

FIG. 5 is a diagram illustrating a method for polishing an edge portionof a glass substrate in an embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a magnetic-disk glass substrate of this embodiment and amethod for manufacturing the same will be described in detail.

[Magnetic-Disk Glass Substrate]

Aluminosilicate glass, soda-lime glass, borosilicate glass, or the likecan be used as a material for a magnetic-disk glass substrate of thepresent embodiment. In particular, aluminosilicate glass can bepreferably used because it can be chemically strengthened and be used toproduce a magnetic-disk glass substrate having excellent flatness of itsmain surfaces and excellent strength of the substrate. Furthermore,amorphous aluminosilicate glass facilitates improving smoothness of thesurface, such as roughness, and is more preferable.

Although there is no limitation on the composition of the glass materialthat is used for the magnetic-disk glass substrate of the presentembodiment, the glass substrate of the present embodiment may preferablybe amorphous aluminosilicate glass containing SiO₂, Li₂O, and Na₂O andone or more alkaline earth metal oxides selected from the group ofconsisting of MgO, CaO, SrO, and BaO as essential components, and havinga molar ratio of the content of CaO to the total content of MgO, CaO,SrO, and BaO (CaO/(MgO+CaO+SrO+BaO)) of 0.20 or less, and a glasstransition point of 650° C. or more.

The glass substrate may also be crystallized glass containing, in termsof oxide amount in mass %, SiO₂ in an amount of 45.60 to 60%, Al₂O₃ inan amount of 7 to 20%, B₂O₃ in an amount of 1.00 to less than 8%, P₂O₅in an amount of 0.50 to 7%, TiO₂ in an amount of 1 to 15%, and RO (itshould be noted that R represents Zn and Mg) in a total amount of 5 to35%, CaO in an amount of 3.00% or less, BaO in an amount of 4% or less,no PbO component, no As₂O₃ component, no Sb₂O₃ component, no Cl⁻component, no NO⁻ component, no SO²⁻ component, no F⁻ component, and oneor more selected from RAl₂O₄ and R2TiO4 (it should be noted that Rrepresents one or more selected from Zn and Mg) as a main crystal phase,in which the particle size of crystals in the main crystal phase is in arange of 0.5 nm to 20 nm, the degree of crystallization is 15% or less,and the specific gravity is 2.95 or less.

The magnetic-disk glass substrate having such a composition has a Tg of650° C. or more and has high heat resistance, and thus is preferable fora magnetic-disk glass substrate to be used for a magnetic disk forenergy-assisted magnetic recording. Also, such a magnetic-disk glasssubstrate is preferable because it is possible to set a coefficient ofthermal expansion (CTE) to 60×10⁻⁷ [K⁻¹] or less. It should be notedthat the coefficient of thermal expansion referred to in thisspecification indicates an average coefficient of thermal expansioncalculated using the rate of thermal expansion of the glass substrate ata temperature of 100° C. and 300° C.

The glass material having the above-described composition is merely anexample. The glass material may have another composition that can beused as the magnetic-disk glass substrate as long as the shape of acircular hole, which will be described later, is satisfied.

FIG. 1A shows an external shape of a magnetic-disk glass substrate 1 ofthe embodiment. As shown in FIG. 1A, the magnetic-disk glass substrate 1according to the present embodiment is a thin doughnut-shaped glasssubstrate having an inner hole 2 (circular hole) at the center. Althoughthere is no limitation on the size of the magnetic-disk glass substrate1, the magnetic-disk glass substrate 1 is preferably formed as amagnetic-disk glass substrate having a nominal diameter of 2.5 inches,for example. It should be noted that the thickness (0.635 mm, 0.8 mm, orthe like) of the magnetic-disk glass substrate referred to in thedescription below is a nominal value and the actual measurement valuemay be slightly thicker or thinner than the nominal value.

FIG. 1B is an enlarged cross-sectional view of an inner circumferentialside edge portion of the magnetic-disk glass substrate 1 of the presentembodiment. As shown in FIG. 1B, the magnetic-disk glass substrate 1 hasa pair of main surfaces 1 p, a side wall surface 1 t arranged along adirection orthogonal to the pair of main surfaces 1 p, and a pair ofchamfered surfaces 1 c arranged between the pair of main surfaces 1 pand the side wall surface 1 t. Although not shown, a side wall surfaceand chamfered surfaces are similarly formed for the outercircumferential side edge portion of the magnetic-disk glass substrate1. It should be noted that the chamfered surface may be formed into acircular arc in cross-sectional view. The side wall surface and thechamfered surfaces are also correctively referred to as an “edgesurface” in the following description. The inner circumferential edgesurface is an inner circumferential edge surface, and the outercircumferential edge surface is an outer circumferential edge surface.

The magnetic-disk glass substrate 1 of the present embodiment isconfigured such that the roundness of an inner hole 2 of the glasssubstrate is 1.5 μm or less, and when outlines of the side wall surface1 t in the circumferential direction at a plurality of differentpositions in the substrate thickness direction including at least threepositions spaced apart by 200 μm in the substrate thickness direction onthe side wall surface 1 t of the inner hole 2 are obtained, amonginscribed circles of the respective outlines, a difference between theradius of the smallest inscribed circle and the radius of the largestinscribed circle is 3. 5 μm or less. It should be noted that when theoutlines of the side wall surface 1 t in the circumferential directionat the plurality of different positions in the substrate thicknessdirection including at least three positions spaced apart by 200 μm inthe substrate thickness direction on the side wall surface 1 t of theinner hole 2 are respectively obtained, among the inscribed circles ofthe respective outlines, the difference between the radius of thesmallest inscribed circle and the radius of the largest inscribed circleis referred to as the “shape evaluation value of the inner hole” or asmerely the “shape evaluation value” hereinafter. As the shape evaluationvalue of the inner hole of the magnetic-disk glass substrate deceases,the inner hole of the magnetic disk, when formed from this glasssubstrate and mounted on the spindle, becomes less likely to locallyabut against the spindle in the substrate thickness direction, or inother words, the side wall surface of the inner hole of the magneticdisk becomes more likely to come into surface contact with the spindle,and this is thought to work advantageously against distortion of themagnetic disk (that is, the magnetic disk is unlikely to becomedistorted) due to contact with the spindle.

It should be noted that the method for measuring roundness can be aknown method. For example, as described above, a plate-shaped probe thatis longer than the thickness of the glass substrate is inserted into theinner hole in the vertical direction with respect to the main surface ofthe glass substrate, an outline is obtained by scanning the inner holein the circumferential direction, and thus a difference in the radiusbetween an inscribed circle and a circumscribed circle of this outlinecan be calculated as the roundness of the glass substrate.

The method for calculating a shape evaluation value of the inner hole ofthe magnetic-disk glass substrate will be described with reference toFIGS. 2A, 2B, and 3. FIGS. 2A and 2B are diagrams illustrating themethod for measuring outlines of the side wall surface 1 t of themagnetic-disk glass substrate 1 of the embodiment, and FIG. 3 is adiagram illustrating the method for calculating a shape evaluation valuebased on the outlines of the side wall surface 1 t of the magnetic-diskglass substrate 1 of the embodiment.

Similarly to FIG. 1B, FIG. 2A is an enlarged cross-sectional view of theinner hole 2of the magnetic-disk glass substrate 1 of the embodiment. InFIG. 2A, a central position C in the substrate thickness direction ofthe inner hole 2of the magnetic-disk glass substrate 1, a position Aupward from the central position C to one of the main surface sides by200 μm, and a position B downward from the central position C to theother of the main surface sides by 200 μm are three examples of aplurality of positions (that is, measurement positions in the substratethickness direction when referenced on either one of the main surfaces)at which the outline of the side wall surface 1 t of the inner hole 2 ismeasured. These positions are favorable for a magnetic-disk glasssubstrate having a thickness of 0.8 mm or more, for example. It shouldbe noted that in the case where the thickness is 0.635 mm or less or themeasurement positions A and C used in the above-described method fordetermining measurement positions will miss the side wall surface due tothe large chamfering amount or the like, the measurement positions A andC may be spaced apart from each other by 100 μm in the substratethickness direction. A stylus 3 is set at these positions A to C toobtain the outlines of the inner hole 2. Accordingly, as shown in FIG.2B, three outlines Pa to Pc are obtained at different heights of theinner hole in the substrate thickness direction. It should be noted thatthe stylus 3 preferably uses a small hole gauge head having a relativelysmall diameter with a radius of curvature of the tip of φ0.4 mm or less,for example, so that measurement can be performed at a plurality ofpositions in the substrate thickness direction. Also, although threemeasurement positions are shown as examples in FIGS. 2A and 2B, thenumber of measurement positions can be greater than 3.

In the magnetic-disk glass substrate 1 of the present embodiment, theroundness of the inner hole 2 is 1.5 μm or less. Also, the inscribedcircles of the three outlines Pa to Pc are obtained, and a shapeevaluation value of the inner hole 2 is measured as follows, based onthe data of three inscribed circles (shown in FIG. 3). It should benoted that the centers of three inscribed circles are located at thesame position.

Referring to FIG. 3, first, radii of the inscribed circles of the threeoutlines Pa to Pc are derived. Next, a difference between a maximumvalue Rmax and a minimum value Rmin of the radii of the inscribedcircles of the three outlines is derived, and the difference of theradii is used as a shape evaluation value of the inner hole 2. In themagnetic-disk glass substrate of the present embodiment, the shapeevaluation value of the inner hole 2 is 3.5 μm or less, and morefavorably 2.5 μm or less.

The surface roughness of the side wall surface 1 t of the inner hole 2(that is, on the inner circumferential side) preferably has a maximumheight Rz of 0.2 μm or less and more preferably 0.1 μm or less. Also,the surface roughness thereof preferably has an arithmetic meanroughness Ra of 0.02 μm or less. Setting Rz and Ra in this range canprevent thermal asperity resulting from adherence or digging in offoreign substances and corrosion resulting from the deposition of ionssuch as sodium and potassium. Also, for similar reasons described above,it is also preferable that the surface roughness of the pair ofchamfered surfaces 1 c is in the above-described range. Here, Rz refersto the maximum height defined by JIS B 0601:2001. Ra refers to thearithmetic mean roughness defined by JIS B 0601:2001.

Also, with regard to the surface roughness of the side wall surface 1 tof the inner hole 2, letting the maximum height in the substratethickness direction be Rz(t) and the maximum height in thecircumferential direction be Rz(c), Rz(t)/Rz(c) is preferably 1.2 orless, and even more preferably 1.1 or less. If Rz(t)/Rz(c) exceeds theabove-described range, variation in the shape evaluation value ofsubstrates is likely to increase at the time of mass production. Bysetting Rz(t)/Rz(c) to a value in the above-described range, it ispossible to reduce variation in the shape evaluation value.

It should be noted that the value of surface roughness can be obtainedby measuring the side wall surface 1 t with a wavelength bandwidth inwhich the surface roughness is measured using a laser microscope setfrom 0.25 μm to 80 μm, for example, and selecting and analyzing a regionof 50 μm square in the measured range. The surface roughness in thesubstrate thickness direction and the circumferential direction can takean average value of data obtained by measuring the line roughness of theregion of 50 μm square, for example, from a plurality of cross-sectionsrespectively corresponding in the substrate thickness direction and thecircumferential direction. For example, it is sufficient that five setsof data are obtained and the average thereof is used as the surfaceroughness.

Next, a preferable shape of a portion between the side wall surface 1 tand the chamfered surface 1 c will be described.

First, a method for deriving the radius of curvature of the shape of aportion between the side wall surface 1 t and the chamfered surface 1 cwill be described with reference to FIG. 4. In FIG. 4, R is a radius ofa circle C2 forming the curvature of the shape of the portion betweenthe side wall surface 1 t and the chamfered surface 1 c, and is theradius of curvature of the shape of the portion. The radius of curvatureR is derived as follows, for example. First, an intersection point of avirtual line L1 obtained by extending a linear portion of the chamferedsurface 1 c and a virtual line L2 obtained by extending a linear portionof the side wall surface 1 t is denoted by P1. Next, a virtual line L3passing through the intersection point P1 and extending perpendicular tothe linear portion of the chamfered surface 1 c is set. Next, anintersection point of the virtual line L3 and the portion between theside wall surface 1 t and the chamfered surface 1 c is denoted by P2.Also, a circle C1 having a predetermined radius (50 μm, for example)around the intersection point P2 is set on the cross-section of themagnetic-disk glass substrate 1. Also, two intersection points of anouter circumference of the circle C1 and the portion between the sidewall surface 1 t and the chamfered surface 1 c are respectively denotedby P3 and P4. Furthermore, a circle C2 respectively passing throughthree intersection points P2, P3, and P4 are set. By deriving the radiusof the circle C2, the radius of curvature R of the shape of the portionbetween the side wall surface 1 t and the chamfered surface 1 c can thenbe derived.

It should be noted that the radii of curvature of shapes of bothportions between the side wall surface 1 t and the chamfered surface 1 cadjacent to one main surface 1 p and between the side wall surface 1 tand the chamfered surface 1 c adjacent to the other main surface 1 p canalso be derived as described above.

In the present embodiment, a measurement point is provided every 30degrees in the circumferential direction, referenced on the center ofthe magnetic-disk glass substrate 1. In other words, the number ofmeasurement points is 12. When the radius of curvature R of the shape ofthe portion between the side wall surface 1 t and the chamfered surface1 c is derived at each measurement point, it is preferable that adifference in the radius of curvature R between adjacent measurementpoints is set to 0.01 mm or less. Accordingly, it is possible to reducea change in the shape of the inner circumferential edge surface in thecircumferential direction of the magnetic-disk glass substrate 1, andreduce variation in the shape evaluation value of the inner hole 2. Itshould be noted that the difference in the radius of curvature R betweenadjacent measurement points is more preferably 0.005 mm or less, becausevariation in the shape evaluation value of the inner hole 2 can befurther reduced.

On the main surface of the magnetic-disk glass substrate 1, a regionincluding locations at which a magnetic disk is clamped by a clampingmember when the magnetic disk produced based on the magnetic-disk glasssubstrate 1 is fixed to the HDD is referred to as a “clamp region”. Theclamp region has a diameter that is 128% of the diameter of the circularhole on the main surface, and is an annular region on the centralportion side of the circumference of a concentric circle with thecircular hole. It is preferable that the clamp region has a flatness of1 μm or less.

The flatness of such a clamp region is expressed by TIR (Total IndicatedRunout) value, which is a difference between a height of the maximumpeak and a depth of the maximum valley. Measurement of flatness can beperformed using an interference flatness measurement device throughphase-measurement interferometry (phase shifting) at a predeterminedmeasurement wavelength, for example. Specifically, it is sufficient thatthe flatness of the clamp region of both main surfaces of the glasssubstrate is measured using a light source having a measurementwavelength of 680 nm through phase-measurement interferometry (phaseshifting). It should be noted that it is sufficient that the flatness ismeasured in the clamp region of both main surfaces and a higher value isused as the measured flatness of the glass substrate.

On the main surface of the glass substrate, if the flatness is low at alocation where the magnetic disk is clamped and fixed by the clampingmember, the shape of the glass substrate slightly deforms when themagnetic disk is clamped using the clamping member, as a result of whichthe overall flatness of the magnetic disk may be deteriorated. As aresult, there are cases where fluttering is deteriorated. Thus, asdescribed above, it is preferable that the flatness of the clamp regionis 1 μm or less.

In the case where a magnetic disk in which a magnetic layer is formed onthe main surface of the magnetic-disk glass substrate of the presentembodiment is produced and integrated into the spindle of a HDD, theroundness and the shape evaluation value of the inner hole of themagnetic disk are extremely small. Therefore, distortion of the magneticdisk resulting from the side wall surface of the magnetic disk on theinner hole side locally abutting against the spindle is unlikely tooccur. For example, in the case where the coefficient of thermalexpansion of the glass substrate is smaller than that of the spindle andthe HDD is placed in a high temperature atmosphere, even in a case wherethe spindle expands relatively largely with respect to the glasssubstrate, the entire side wall surface of the magnetic disk on theinner hole side comes into contact with the spindle, and therefore theabove-described distortion is unlikely to occur. Thus, when the magneticdisk of the HDD is rotated at high speed, the level of fluttering isunlikely to increase. Although the coefficient of thermal expansion ofthe spindle is about 90 to 100×10⁻⁷ [K⁻¹] or more, for example, themagnetic-disk glass substrate of the present embodiment is favorable inthe case in which a difference in the coefficient of thermal expansionbetween the spindle and the glass substrate increases. In particular,the magnetic-disk glass substrate of the present embodiment is favorablein the case of a magnetic-disk glass substrate having a coefficient ofthermal expansion of 60×10⁻⁷ [K⁻¹] or less or the like to be used for amagnetic disk for energy-assisted magnetic recording.

When a magnetic disk on which a magnetic layer having a track recordingdensity of 500 kTPI (tracks per inch) or more, in particular, is formedis integrated into the HDD, such as a magnetic disk for energy-assistedmagnetic recording, there are cases where accuracy in positioning themagnetic head at the data track of the HDD deteriorates due to slightdistortion in the magnetic disk, and thus the magnetic-disk glasssubstrate of the present embodiment is favorable for a magnetic diskprovided with the above-described high recording density.

[Method for Manufacturing Magnetic-Disk Glass Substrate]

Hereinafter, a method for manufacturing the magnetic-disk glasssubstrate of the present embodiment will be described step-by-step. Itshould be noted that the order of the steps may be changed asappropriate.

(1) Glass Substrate Formation

A raw glass plate is molded by press molding and processes areappropriately performed to form an inner hole and an outer shape toobtain an annular glass substrate having an inner hole that has apredetermined thickness, for example. It should be noted that the methodfor molding a raw glass substrate is not limited to these methods and aglass substrate can also be manufactured by a known manufacturing methodsuch as a float method, a down draw method, a redraw method, or a fusionmethod.

(2) Edge Surface Grinding Step

Next, the edge surfaces of the annular glass substrate are ground. Theedge surfaces of the glass substrate are ground in order to formchamfered surfaces at the outer circumferential side edge portion andthe inner circumferential side edge portion of the glass substrate, andadjust the outer and inner diameter of the glass substrate. The grindingprocessing performed on the outer circumferential side edge surface ofthe glass substrate may be known chamfering processing with a formedgrindstone using diamond abrasive particles, for example.

The inner circumferential side edge surface of the glass substrate isground using a formed grindstone and by additional grinding processingin which a grindstone is brought into contact with the edge surface ofthe glass substrate such that a locus of the grindstone that is incontact with the edge surface of the glass substrate is not constant.Hereinafter, the additional grinding processing on the innercircumferential side edge surface of the glass substrate will bedescribed with reference to FIG. 5.

FIG. 5 is a diagram showing a method for processing the innercircumferential side edge surface of the glass substrate.

As shown in FIG. 5, a grindstone 40 used to grind the innercircumferential side edge surface of the glass substrate G is formed ina cylindrical shape as a whole and has a groove 50. The groove 50 isformed so as to be capable of simultaneously grinding both the side wallsurface 1 t and the chamfered surface 1 c of the glass substrate G onthe inner circumferential side. Specifically, the groove 50 has a grooveshape including a side wall portion 50 a and chamfering portions 50 blocated on both sides of the side wall portion 50 a. The side wallportion 50 a and the chamfering portions 50 b of the groove 50 describedabove are formed so as to have predetermined dimensions and shapes inconsideration of the finishing target dimensions and shapes of theground surfaces of the glass substrate G.

In the processing of the inner circumferential side edge surface of theglass substrate, the grinding processing is performed by rotating boththe glass substrate G and the grindstone 40 while bringing thegrindstone 40 into contact with the inner circumferential side edgesurface 1 t of the glass substrate G in a state in which the glasssubstrate G is inclined with respect to the groove direction of thegroove 50 formed in the grindstone 40, that is, in a state in which arotation axis L₁ of the glass substrate G is inclined by an angle α (inFIG. 5, α is a positive counterclockwise angle) with respect to arotation axis L₄₀ of the grindstone 40. Accordingly, the locus of thegrindstone 40 that abuts against the inner circumferential side edgesurface of the glass substrate G is not constant, and the abrasiveparticles of the grindstone 40 abut against and act on the edge surfaceof the substrate at random positions. Therefore, since impairment of thesubstrate is reduced, the surface roughness of the ground surface isreduced, and in-plane variation is reduced, it is possible to make theground surface smoother, that is, to finish the ground surface with aquality of a level that meets the requirement for higher quality.Furthermore, the effect of improving the life of the grindstone isobtained.

Moreover, as shown in FIG. 5, the grindstone 40 and the glass substrateG are in contact with each other in a state in which the groove 50 ofthe grindstone 40 and an inner diameter arc of the glass substrate G arein contact with each other in a surface contact state, thus increasing acontact area between the grindstone 40 and the glass substrate G.Therefore, a contact length (cutting blade length) of the grindstone 40with respect to the glass substrate G is extended, thus making itpossible to maintain the sharpness of the abrasive particles.Accordingly, stable grinding performance can be secured even in the casewhere the grinding processing is performed using a grindstone with fineabrasive particles that is advantageous in terms of the quality of theground surface, and the favorable quality of the ground surface (mirrorsurface quality) can be stably obtained by grinding processing mainlyusing a plastic mode. In addition, the sharpness of the grindstone ismaintained and the grinding performance for achieving the plastic modeis stably secured, thus making it possible to secure the favorableaccuracy of dimensions and shapes obtained by chamfering processingperformed on the inner circumferential edge surface of the glasssubstrate.

Although the inclination angle α of the glass substrate G with respectto the groove direction of the grindstone 40 described above can be setarbitrarily, it is preferable that the inclination angle α is in a rangeof one to fifteen degrees in order to more favorably exhibit theoperations and effects described above. It is preferable that thegrindstone 40 used in the grinding processing is a grindstone obtainedby binding diamond abrasive particles with resin (resin bondgrindstone). It is preferable to use a 2000# to 3000# diamondgrindstone.

A preferable example of the circumferential speed of the grindstone 40is 500 to 3000 in/minute, and the circumferential speed of the glasssubstrate G is about 1 to 30 m/minute. In addition, it is preferablethat the ratio (circumferential speed ratio) of the circumferentialspeed of the grindstone 40 to the circumferential speed of the glasssubstrate G is in a range of 50 to 300.

It should be noted that the above-described grinding step can be dividedinto two steps, and first grinding is performed in a state in which therotation axis of the glass substrate G is inclined by an angle α (α>0),as described above, second grinding is performed in a state in which therotation axis of the glass substrate G is inclined by an angle −α usinganother grindstone, and adjustment is performed such that the machiningallowance of the second grinding is smaller than the machining allowanceof the first grinding, as a result of which Rz(t)/Rz(c) can be 1.2 orless.

It is preferable that the hardness (referred to as “grindstone hardness”hereinafter) obtained by measuring a binder (resin) portion on thegrindstone surface of the above-described resin bond grindstone using aBerkovich indenter under conditions where an indentation load is 250 mNby a nanoindentation test method is in a range of 0.4 to 1.7 GPa. In thecase of the resin bond grindstone, the grindstone hardness is an indexthat is correlated with a bond strength between the diamond abrasiveparticles and the resin.

As a result of grinding the inner circumferential side edge surfaceusing resin bond grindstones having various characteristics andobserving the processed quality of the edge surface of the glasssubstrate, the inventors of the present invention found that the bondstrength between the diamond abrasive particles and the resin in theresin bond grindstone had a large influence on the shape evaluationvalue of the inner hole of the glass substrate subjected to theabove-described grinding processing. That is, it was found that if theinner circumferential side edge surface is ground using a resin bondgrindstone having a grindstone hardness that is too high, the processingrate is favorable but the surface is likely to be blemished and theshape evaluation value of the inner hole is deteriorated, whereas if theinner circumferential side edge surface is ground using a resin bondgrindstone having a grindstone hardness that is too low, the shapeevaluation value of the inner hole is favorable but the processing ratedecreases markedly. In other words, the shape evaluation value of theinner hole of the glass substrate can be adjusted by changing thegrindstone hardness. As a result, it was found that the grindstonehardness was preferably in the above-described range. By setting thegrindstone hardness in the above-described range, it is possible toprocess the inner circumferential side edge surface subjected to thegrinding processing to a semi-mirror surface, and therefore, themachining allowance can be reduced in a subsequent edge surfacepolishing step, thus making it possible to improve the shape accuracy ofthe edge portion including the shape evaluation value of the inner holewhile maintaining high surface quality.

A method for measuring grindstone hardness by a nanoindentation testmethod will be described. A load is applied at 1 nm/sec to a binderportion of the grindstone surface, which is the measurement target,using a Berkovich indenter having a quadrangular pyramidal tip, thepressure is increased to 250 mN and held for a predetermined time (10seconds, for example), and then a relationship between the load and thedisplacement when the pressure is reduced at an unloading rateequivalent to when the pressure was increased is obtained. A curveobtained here indicates dynamic hardness, which is a characteristiccloser to actual use conditions than evaluation of hardness, which is aconventional static hardness characteristic. Based on the result of theobtained curve of dynamic hardness characteristics, grindstone hardnesscan be obtained by the nanoindentation test method using Equation (1)below.

H=F/Ac   (1)

where H is the hardness of the grindstone, F is a load, and Ac is anindentation area.

The above-described indentation area Ac is expressed by relationalexpressions (2) and (3) below.

Ac=f(hc)∝24.5 ·hc ²   (2)

hc=hmax−ε·F/S   (3)

where he is an indentation depth, hmax is a depth at maximum load, hs isan indentation depth at the start of unloading, ho is an indentationdepth after unloading, ε is a shape coefficient specific to the indenter(example: in case of a Berkovich indenter=0.75), S is a proportionalitycoefficient of the load and displacement, and m is a slope (dF/dh).

(3) Edge Surface Polishing Step

Next, the edge surfaces of the annular glass substrate are polished. Theedge surfaces of the glass substrate are polished in order to improvethe properties of the outer circumferential side edge surface and theinner circumferential side edge surface (side wall surface and chamferedsurfaces) of the glass substrate. In the edge surface polishing step,the outer circumferential side edge surface and the innercircumferential side edge surface of the glass substrate are polished bybrushing.

By performing the edge surface grinding and the edge surface polishingdescribed above, contamination by attached waste and the like andimpairment such as scratches on the edge surface of the glass substratecan be eliminated, thermal asperity and deposition of ions such assodium and potassium that causes corrosion can be prevented, and surfaceroughness and waviness can also be significantly reduced and the shapeevaluation value of the inner hole of the glass substrate can bereduced, thus making it possible to improve the shape accuracy of theedge portion of the inner hole.

(4) First Polishing (Main Surface Polishing) Step

After the main surface grinding step is appropriately performed asrequired, first polishing is performed on the ground main surfaces ofthe glass substrate. The first polishing is performed in order toeliminate scratches and distortions that remain on the main surfaces dueto the main surface grinding or the like and to adjust surfaceunevenness (microwaviness, roughness).

In the first polishing step, the main surfaces of the glass substrateare polished using a double-side polishing device provided with aplanetary gear mechanism. The double-side polishing device has an uppersurface plate and a lower surface plate. Planar polishing pads (resinpolishers) are attached to the upper surface of the lower surface plateand the bottom surface of the upper surface plate. One or more glasssubstrates accommodated in a carrier are held between the upper surfaceplate and the lower surface plate, and the glass substrate and thesurface plates are moved relative to each other by the planetary gearmechanism moving one or both of the upper surface plate and the lowersurface plate while supplying loose abrasive particles including anabrasive, so that the two main surfaces of the glass substrate can bepolished.

During the relative motion described above, the upper surface plate ispressed against the glass substrate (that is, in a vertical direction)with a predetermined load, the polishing pads are pressed against theglass substrate, and a polishing liquid is supplied between the glasssubstrate and the polishing pads. The main surfaces of the glasssubstrate are polished by the abrasive contained in this polishingliquid. Known abrasive particles such as Cerium oxide, zirconium oxide,and silicon dioxide can be used as the abrasive, for example. It shouldbe noted that this step may be divided into a plurality of stepschanging the type or size of the abrasive particles.

(5) Chemical Strengthening Step

Furthermore, as required, the glass substrate may be chemicallystrengthened. A molten liquid of mixed salts of potassium nitrate andsodium nitrate, for example, can be used as a chemical strengtheningliquid. Chemical strengthening processing is performed by immersing theglass substrate in the chemical strengthening liquid, for example. Inthis manner, by immersing the glass substrate in the chemicalstrengthening liquid, lithium ions and sodium ions in the surface layerof the glass substrate are respectively substituted with sodium ions andpotassium ions with a relatively large ion radius in the chemicalstrengthening liquid, and the glass substrate is strengthened.

(6) Second Polishing (Final Polishing) Step

Next, second polishing is performed on the glass substrate. The secondpolishing is performed in order to mirror polish the main surfaces. Inthe second polishing, a polishing device used in the first polishing isused, for example. In this case, the second polishing differs from thefirst polishing in the type and size of loose abrasive particles and thehardness of the resin polisher.

Microparticles (particle size: diameter of about 10 to 100 m) ofcolloidal silica or the like suspended in a slurry, for example, areused as the loose abrasive particles to be used in the second polishing.This makes it possible to further reduce the surface roughness of themain surfaces of the glass substrate and to adjust the shape of the edgeportion in a preferable range. The polished glass substrate is cleanedto provide a magnetic-disk glass substrate.

[Magnetic disk]

A magnetic disk can be obtained as follows using the magnetic-disk glasssubstrate. A magnetic disk has a configuration in which at least anadherent layer, a base layer, a magnetic layer (magnetic recordinglayer), a protecting layer and a lubricant layer are laminated on themain surface of the magnetic-disk glass substrate (referred to as merely“substrate” hereinafter) in this order from the main surface side, forexample.

For example, the substrate is introduced into a film deposition devicethat has been evacuated and the layers from the adherent layer to themagnetic layer are sequentially formed on the main surface of thesubstrate in an Ar atmosphere by a DC magnetron sputtering method. CrTican be used in the adherent layer and CrRu can be used in the baselayer, for example. A CoPt-based alloy can be used in the magneticlayer, for example. Also, a CoPt-based alloy or a FePt-based alloyhaving an L₁₀ ordered structure is formed as the magnetic layer forthermally assisted magnetic recording. After the film deposition asdescribed above, by forming the protecting layer using C₂H₄ by a CVDmethod, for example, and subsequently performing nitriding processingthat introduces nitrogen to the surface, a magnetic recording medium canbe formed. Thereafter, by coating the protecting layer withperfluoropolyether (PFPE) by a dip coat method, the lubricant layer canbe formed.

The produced magnetic disk is preferably incorporated in a magnetic-diskdrive device (hard disk drive (HDD)) serving as a magnetic recording andreproduction device provided with a magnetic head equipped with adynamic flying height (DFH) control mechanism and a spindle for fixingthe magnetic disk.

WORKING EXAMPLES AND COMPARATIVE EXAMPLES

In order to confirm the effect of the magnetic-disk glass substrate ofthe present embodiment, 2.5-inch magnetic disks (having an outerdiameter of 65 mm, an inner diameter of 20 mm, and a thickness of 0.8mm, a length of a side wall surface on the inner diameter side of 0.5mm, and an angle of the chamfered surface of 45 degrees with respect tothe main surface) were produced using manufactured magnetic-disk glasssubstrates. The glass composition of the produced magnetic-disk glasssubstrate was as follows.

(Glass Composition)

Amorphous aluminosilicate glass was used that contained SiO₂ in anamount of 63 mol %, Al₂O₃ in an amount of 10 mol %, Li₂O in an amount of1 mol %, Na₂O in an amount of 6 mol %, MgO in an amount of 19 mol %, CaOin an amount of 0 mol %, SrO in an amount of 0 mol %, BaO in an amountof 0 mol %, and ZrO₂ in an amount of 1 mol %.

It should be noted that the molar ratio of the content of CaO to thetotal content of MgO, CaO, SrO and BaO (CaO/(MgO+CaO+SrO+BaO)) was zero,and the glass-transition temperature was 703° C. The coefficient ofthermal expansion of the glass material having this composition was56×10⁻⁷ [K⁻¹].

PRODUCTION OF MAGNETIC-DISK GLASS SUBSTRATES OF WORKING EXAMPLES ANDCOMPARATIVE EXAMPLES

The magnetic-disk glass substrates of working examples were produced byperforming the steps of the method for manufacturing a magnetic-diskglass substrate according to the present embodiment in the given order.Here, the press molding method was used in molding of the glasssubstrate, and an inner diameter and an outer shape were formed, and thethickness was adjusted using a known method.

In the edge surface grinding step, the inner circumferential edgesurface and the outer circumferential edge surface of the glasssubstrate were chamfered and the inner circumferential side wall surfaceand the outer circumferential side wall surface were ground with aformed grindstone using diamond abrasive particles to form chamferedsurfaces and a side wall surface. Furthermore, with regard to the innercircumferential side edge surface of the glass substrate, by addinggrinding processing in which the edge surface of the glass substrate isinclined and brought into contact with the grindstone such that thelocus of the grindstone abutting against the edge surface of the glasssubstrate was not constant, surface quality was further improved whileincreasing the shape accuracy of the chamfered surfaces and the sidewall surface.

In the additional grinding processing performed on the innercircumferential side edge surface of the glass substrate, a resin bondgrindstone with 2500# diamond abrasive particles was used, and theinclination angle (α in FIG. 5) of the glass substrate with respect tothe groove direction of the grindstone was set to 5 degrees. Otherconditions were adjusted as appropriate. In this case, glass substratesthat were different in the shape evaluation value of the inner hole wereproduced by adjusting the inclination angle (α in FIG. 5) of the glasssubstrate with respect to the groove direction of the grindstone and theother factors (e.g., grit of the grindstone, and circumferential speedof the grindstone or the glass substrate) in the above-described rangeas appropriate. It should be noted that although in the case of aworking example 1 in Table 1, α=5 degrees, by further increasing theinclination angle, the surface quality is improved after grinding, andthus it is possible to further improve the shape evaluation value.

It should be noted that in case of the working example 1 of Table 1, aresin bond grindstone having a grindstone hardness of 1.05 GPa was usedto perform edge surface grinding.

In the edge surface polishing step, the brushing was performed on theinner circumferential side edge surface and the outer circumferentialside edge surface of the glass substrate, using a slurry containingcerium oxide abrasive particles as polishing abrasive particles. Itshould be noted that the machining allowance for a chamfered surface inthe edge surface polishing was adjusted in accordance with the surfacequality after the edge surface grinding step as appropriate.

Thereafter, grinding was performed on the main surface using a knownmethod, and then two-step polishing and chemical strengthening wereperformed thereon. A polishing liquid containing cerium oxide abrasiveparticles was used in the first polishing, and a polishing liquidcontaining colloidal silica polishing abrasive particles was used in thesecond polishing. The chemical strengthening was performed before thesecond polishing. The glass substrate on which polishing has beenperformed was cleaned using a known cleaning method as appropriate.Accordingly, the magnetic-disk glass substrate was obtained.

Through the above steps, samples of the magnetic-disk glass substratesof the working examples and comparative examples were produced as shownin Table 1. As shown in Table 1, the magnetic-disk glass substrates ofthe working examples and comparative examples were different from eachother in the shape evaluation value of the inner hole. As describedabove, the glass substrates that are different in the shape evaluationvalue of the inner hole were produced mainly by adjusting theinclination angle of the glass substrate with respect to the groovedirection of a grindstone used in the grinding processing performed onthe inner circumferential side edge surface of the glass substrate asappropriate. Although not shown in each table below, the roundness ofthe samples of the working examples and comparative examples was 1.5 μmor less.

It should be noted that a plate-shaped probe that was longer than thethickness of the produced magnetic-disk glass substrate was insertedinto the inner hole in the vertical direction with respect to the mainsurface of the glass substrate, an outline was obtained by scanning theinner hole in the circumferential direction, and thus the roundness ofthe inner hole of the magnetic-disk glass substrate was calculated as adifference in the radius between an inscribed circle and a circumscribedcircle of this outline. The shape evaluation value of the inner hole wascalculated based on three outlines obtained at positions shown in FIG.2A. That is, outlines were obtained at the central position of the innerhole in the substrate thickness direction and the positions spaced apartupward and downward by 200 μm from the central position, and among theinscribed circles of the three outlines, the difference between theradius of the smallest inscribed circle and the radius of the largestinscribed circle was used as the shape evaluation value of the innerhole. All measurements were performed using a roundness/cylindricalshape measurement machine.

[Evaluation Method]

Next, the samples of the magnetic-disk glass substrates of the workingexamples and comparative examples were formed into films as describedabove to produce samples of magnetic disks of working examples andcomparative examples. Fluttering was evaluated by measuring flutteringcharacteristic values of the samples of the magnetic disks of thecomparative examples and the working examples using a laser Dopplervibrometer. In the measurement of the fluttering characteristic value, amagnetic disk was mounted on the spindle of a 2.5-inch type HDD and wasrotated, and the main surface of the rotating magnetic disk wasirradiated with a laser beam from a laser Doppler vibrometer. It shouldbe noted that the cover of the HDD was provided with a hole for laserbeam irradiation. Next, the laser Doppler vibrometer received the laserbeam reflected by the magnetic disk, and thus the amount of vibration inthe thickness direction of the magnetic disk was measured as afluttering characteristic value. In this case, the flutteringcharacteristic values were measured under the following conditions.

Environment for HDD and measurement system: The temperature was kept at80° C. in a constant temperature and humidity chamber.

Rotation rate of magnetic disk: 7200 rpm

Laser beam irradiation position: Position 31 mm apart from the center(1.5 mm apart from the outer circumferential edge) of a magnetic disk inthe radial direction

[Evaluation Criterion]

As described below, the results of evaluation of the measured flutteringcharacteristic values were divided to four levels 1 to 4 in descendingorder of favorability (that is, in increasing order of the flutteringcharacteristic value). Levels 1 and 2 are acceptable for practicalpurposes for a HDD of 500 kTPI.

Level 1: 20 nm or less

Level 2: more than 20 nm to 30 nm or less

Level 3: more than 30 nm to 40 nm or less

Level 4: more than 40 nm

TABLE 1 Shape evaluation value of inner hole (μm) Flutteringcharacteristic value Comp. Ex. 1 5.7 Level 4 Comp. Ex. 2 4.0 Level 3Work. Ex. 1 3.5 Level 2 Work. Ex. 2 2.8 Level 2 Work. Ex. 3 2.5 Level 1Work. Ex. 4 1.9 Level 1

As shown in the comparative examples 1 and 2 of Table 1, in the casewhere the shape evaluation value of the inner hole was not sufficientlysmall even with a magnetic-disk glass substrate in which the roundnessof the inner hole was favorable, fluttering characteristics were notfavorable. It is conceivable that this is because the magnetic diskslightly distorted due to the side wall surface of the magnetic disk onthe inner hole side locally abutting against the spindle. On the otherhand, as shown in the working examples 1 to 4 of Table 1, in the casewhere both the roundness of the inner hole and the shape evaluationvalue were 3.5 μm or less, the fluttering characteristics of the HDDwere favorable. It is conceivable that this is because the magnetic diskdid not distort, due to surface contact between the side wall surface ofthe magnetic disk on the inner hole side and the spindle, even in a hightemperature atmosphere in which a gap between the inner hole of themagnetic disk and the spindle was reduced. It should be noted that asshown in the working examples 3 and 4 of Table. 1, in the case where theshape evaluation value of the inner hole of the magnetic disk was 2.5 μmor less, it was confirmed that fluttering characteristics were furtherimproved. As shown in the working examples 1 to 4, it is conceivablethat in the case where fluttering characteristics were favorable, anerror was unlikely to occur when a magnetic signal was written to orread out from the magnetic disk of the HDD, positioning accuracy by theservo of the HDD was favorable.

It should be noted that when a magnetic-disk glass substrate having aroundness of 1.8 μm and a shape evaluation value of 3.5 μm or 2.5 μm(respectively, comparative examples 3 and 4) were prepared andfluttering characteristic values were measured using the glasssubstrate, each value was level 4. According to this, it was found thateven in the case where the shape evaluation value was 3.5 μm or less, ifthe roundness exceeded 1.5 μm, the level of fluttering was not improved.

Next, ten magnetic-disk glass substrates of the working example 1described above and ten magnetic-disk glass substrates of the workingexamples 5 and 6 were produced, and Rz, Ra, an average value ofRz(t)/Rz(c), and variation in the shape evaluation value were derived.Rz of each glass substrate was 0.2 μm or less. Also, Ra of each glasssubstrate was 0.02 μm or less. The magnetic-disk glass substrates of theworking examples 5 and 6 were produced under the production conditionsof the working example 1 except that only the edge surface grinding stepwas different. Specifically, in the working examples 5 and 6, in theedge surface grinding step, the first grinding was performed such thatthe inclination angle (α in FIG. 5) of the glass substrate with respectto the groove direction of the grindstone was 5 degrees, the secondgrinding was then performed such that the inclination angle of the glasssubstrate was −5 degrees using another grindstone, and adjustment wasperformed such that the machining allowance of the second grinding wassmaller than the machining allowance of the first grinding. Theevaluation results of the working examples 1, 5, and 6 were shown inTable 2. In Table 2, the average value of Rz(t)/Rz(c) is the averagevalue of Rz(t)/Rz(c) of the ten magnetic-disk glass substrates, and“variation in the shape evaluation value” is a difference between themaximum value and the minimum value of shape evaluation values of theten magnetic-disk glass substrates.

It could be found from Table 2 that Rz(t)/Rz(c) was 1.2 or less, andthus variation in the shape evaluation value decreased. Also, it couldbe found that if Rz(t)/Rz(c) was 1.1 or less, variation in the shapeevaluation value further decreased.

TABLE 2 Average value Variation in shape evaluation value of Rz(t)/Rz(c)(μm) Work. Ex. 1 1.31 0.5 Work. Ex. 5 1.17 0.3 Work. Ex. 6 1.08 0.2

Next, ten samples (working examples 7 and 8) were produced under theproduction conditions of the working example 1 except that the machiningallowance of the edge surface grinding was changed, and variation in theshape evaluation value of the working examples 7 and 8 was derived.Similarly to those shown in Table 2, variation in the shape evaluationvalue is a difference between the maximum value and the minimum value ofshape evaluation values of the ten samples.

Also, with regard to the working examples 1, 7, and 8, the radius ofcurvature of a portion between the side wall surface and the chamferedsurface of the inner circumferential edge portion was derived. It shouldbe noted that the shape trimmed in the grinding step is furthermaintained as the machine allowance of the edge surface polishingdecreases, and thus shape accuracy can be increased. In other words, adifference in the radius of curvature can be reduced at adjacentmeasurement positions in the circumferential direction of the innercircumferential edge portion.

The radius of curvature of one glass substrate was derived as follows.Specifically, 24 points of the inner circumferential edge portion,namely 12 points on the surface side and 12 points on the back side,were measured in total. Then, a difference in the radius of curvaturebetween adjacent measurement points in the 12 points on the surface side(twelve sets of data) and a difference in the radius of curvaturebetween adjacent measurement points in the 12 points on the back side(twelve sets of data) were derived, and among twenty-four sets of datain total, the maximum value was used as the maximum value of the radiusof curvature of the glass substrate. Examples of measurement data areshown in Table 3. In Table 3, the surface and the back of the glasssubstrate, which was the measurement target, are respectively indicatedas an A surface and a B surface. Also, in Table 3, a difference in theradius of curvature when “0 to 30 degrees” means the absolute value ofdifferences in the radii of curvature between a measurement point at 0degrees and a measurement point at 30 degrees, for example. Also, theposition of the A surface at 30 degrees on the back side wascorresponded to the position of the B surface at 30 degrees, forexample.

When the maximum value of a difference in the radius of curvature wasderived with regard to the ten samples of the working examples 1, 7, and8, the ten samples of the working example 1 had a maximum value of 0.010mm or less, the ten samples of the working example 7 had a maximum valueof 0.005 mm or less, and the ten samples of the working example 8 had amaximum value of 0.012 mm or less. Examples of measurement data shown inTable 3 are data of one sample having the largest maximum value ofdifferences in the radii of curvature of the working examples.

With regard to the working examples 1, 7, and 8, Table 4 shows themaximum value of differences in radii of curvature (same as the valueindicated in Table 3; the maximum value of the ten samples) andvariation in the shape evaluation value. It could be found from Table 4that by setting the maximum value of differences in radii of curvatureto 0.01 mm or less, variation in the shape evaluation value could besignificantly reduced.

TABLE 3 Difference in radius of curvature (mm) Work. Ex. 1 Work. Ex. 7Work. Ex. 8 A B A B A B surface surface surface surface surface surface0 to 30 0.005 0.009 0.003 0.003 0.006 0.008 degrees 30 to 60 0.003 0.0090.002 0.004 0.003 0.004 degrees 60 to 90 0.005 0.010 0.002 0.003 0.0100.002 degrees 90 to 120 0.004 0.009 0.004 0.004 0.009 0.003 degrees 120to 150 0.006 0.009 0.002 0.003 0.011 0.001 degrees 150 to 180 0.0050.007 0.003 0.002 0.004 0.008 degrees 180 to 210 0.005 0.008 0.001 0.0030.004 0.002 degrees 210 to 240 0.002 0.007 0.001 0.002 0.010 0.003degrees 240 to 270 0.007 0.006 0.004 0.004 0.003 0.005 degrees 270 to300 0.006 0.008 0.003 0.004 0.004 0.007 degrees 300 to 330 0.005 0.0060.005 0.005 0.012 0.011 degrees 330 to 360 0.003 0.007 0.005 0.003 0.0090.010 degrees Max. value of 0.010 0.005 0.012 differences

TABLE 4 Max. value of differences of radii Variation in shape ofcurvature (mm) evaluation value (μm) Work. Ex. 1 0.010 0.3 Work. Ex. 70.005 0.2 Work. Ex 8 0.012 0.6

Also, similarly to the above, a 2.5-inch magnetic-disk glass substratewas produced such that the thickness and the length of the side wallsurface on the inner diameter side were changed respectively to 0.635 mmand 0.335 mm. The roundness of the inner hole of this glass substratewas 1.5 μm or less. Also, when the shape evaluation values of the innerhole of the glass substrate were measured similarly to the above exceptthat three outlines of the inner hole of this glass substrate werespaced apart by 100 μm, the shape evaluation value was 3.4 μm. Similarlyto the above, when this glass substrate was used to produce a magneticdisk and fluttering characteristics thereof were evaluated, thefluttering level was 2.

Furthermore, with regard to the magnetic-disk glass substrate having athickness of 0.635 mm, glass substrates that were different in the shapeevaluation value of the inner hole were produced (comparative example 5and working examples 9 and 10) by adjusting the inclination angle (α inFIG. 5) of the glass substrate with respect to the groove direction ofthe grindstone and the other factors (e.g., grit of the grindstone, andcircumferential speed of the grindstone or the glass substrate) asappropriate. It should be noted that the working examples 9 and 10 wereproduced by increasing the above-described α=5 degrees as a referencesuch that the working examples 9 and 10 were different in the shapeindex value. Measurement results of the comparative example 5 andworking examples 9 and 10 are shown in Table 5.

As shown in Table 5, in the case where the thickness was 0.635 mm, itwas confirmed that if the shape evaluation value of the inner hole ofthe magnetic disk was 3.5 μm or less, the fluttering characteristics ofthe HDD were favorable, whereas if the shape evaluation value was 2.5 μmor less, the fluttering characteristics were further improved.

TABLE 5 Shape evaluation value of inner Fluttering hole (μm)characteristic value Comp. Ex. 5 3.9 Level 3 Work. Ex. 9 2.3 Level 1Work. Ex 1.7 Level 1 10

Next, when the flatness of the clamp region of the main surface of themagnetic-disk glass substrate of the working example 1 was measured, theflatness was 1.1 μm. Also, when the magnetic-disk glass substrate of theworking example 1 was produced, the parallelism of the polishing surfaceof the upper and lower grinding surface plates was reduced during mainsurface polishing, as a result of which the flatness of the clamp regionwas 0.7 μm. It should be noted that the parallelism of the polishingsurfaces (polishing surfaces of polishing pads attached to the upper andlower surface plates) of the upper and lower surface plates was derivedas follows. Specifically, when the distance between the polishingsurface of the upper surface plate and the polishing surface of thelower surface plate in the inner circumferential edge portions of theupper and lower surface plates was D1 and the distance between thepolishing surface of the upper surface plate and the polishing surfaceof the lower surface plate in the outer circumferential edge portion wasD2, the absolute value of (D2-D1) was parallelism.

When the fluttering characteristic value was measured using a magneticdisk substrate, which was based on the magnetic-disk glass substrate inwhich the clamp region had a flatness of 0.7 μm, the flatness of theclamp region was reduced by approximately 10% compared to a case of the1.1 μm. In other words, it was confirmed that when the flatness of theclamp region was reduced, fluttering was improved.

While the magnetic-disk glass substrate and the magnetic disk accordingto the present invention have been described in detail, the presentinvention is not limited to the above-described embodiment, and it willbe appreciated that various improvements and modifications can be madewithout departing from the gist of the present invention.

What is claimed is:
 1. An annular substrate used as an original for amagnetic-disk substrate having a circular hole at a center, andcomprising a pair of main surfaces and a side wall surface orthogonal tothe main surfaces, a roundness of the circular hole being 1.5 μm orless, in the side wall surface of the circular hole, three outlines in acircumferential direction of the side wall surface, which includes anoutline at a center position of a thickness of the substrate andoutlines at two positions that are spaced apart from the center positionin opposite directions along a substrate thickness direction by apredetermined distance, being obtained, a difference between a maximumvalue and a minimum value of radii of three inscribed circles that arerespectively derived from the three outlines being 3.5 μm or less, andwhen positions spaced apart from the center position in the oppositedirections along the substrate thickness direction by 200 μm exist onthe side wall surface, the predetermined distance being 200 μm, and whenpositions spaced apart from the center position in the oppositedirections of the substrate thickness direction by 200 μm do not existon the side wall surface, the predetermined distance being 100 μm. 2.The annular substrate according to claim 1, wherein a substratethickness of the annular substrate is 0.635 mm or less.
 3. The annularsubstrate according to claim 1, wherein a surface roughness Rz of theside wall surface of the circular hole is 0.2 μm or less.
 4. The annularsubstrate according to claim 1, wherein an average coefficient ofthermal expansion from 100° C. to 300° C. is 60×10⁻⁷[K⁻¹] or less. 5.The annular substrate according to claim 1, wherein with regard to thesurface roughness of the side wall surface of the circular hole, in acase where a maximum height in the substrate thickness direction isRz(t) and a maximum height in the circumferential direction is Rz(c),Rz(t)/Rz(c) is 1.2 or less.
 6. The annular substrate according to claim1, wherein when a measurement point is provided every 30 degrees in thecircumferential direction referenced on the center of the annularsubstrate, and a radius of curvature of a shape of a portion between theside wall surface and a chamfered surface of the circular hole at themeasurement point is derived, a difference in the radius of curvaturebetween adjacent measurement points is 0.01 mm or less.
 7. A method formanufacturing a magnetic-disk substrate, the method comprising:polishing at least the main surfaces of the annular substrate accordingto claim
 1. 8. A method for manufacturing a magnetic disk, the methodcomprising: forming at least a magnetic layer on a main surface of themagnetic-disk substrate obtained from the method according to claim 7.9. A magnetic-disk substrate having a circular hole at a center, andcomprising a pair of main surfaces and a side wall surface orthogonal tothe main surfaces, a roundness of the circular hole being 1.5 μm orless, in the side wall surface of the circular hole, three outlines in acircumferential direction of the side wall surface, which includes anoutline at a center position of a thickness of the substrate andoutlines at two positions that are spaced apart from the center positionin opposite directions along a substrate thickness direction by apredetermined distance being obtained, a difference between a maximumvalue and a minimum value of radii of three inscribed circles that arerespectively derived from the three outlines being 3.5 μm or less, andwhen positions spaced apart from the center position in the oppositedirections along the substrate thickness direction by 200 μm exist onthe side wall surface, the predetermined distance being 200 μm, and whenpositions spaced apart from the center position in the oppositedirections of the substrate thickness direction by 200 μm do not existon the side wall surface, the predetermined distance being 100 μm. 10.The magnetic-disk substrate according to claim 9, wherein a substratethickness of the magnetic-disk substrate is 0.635 mm or less.
 11. Themagnetic-disk substrate according to claim 9, wherein a surfaceroughness Rz of the side wall surface of the circular hole is 0.2 μm orless.
 12. The magnetic-disk substrate according to claim 9, wherein anaverage coefficient of thermal expansion from 100° C. to 300° C. is60×10⁻⁷ [K⁻¹] or less.
 13. The magnetic-disk substrate according to anyof claim 9, wherein with regard to the surface roughness of the sidewall surface of the circular hole, in a case where a maximum height inthe substrate thickness direction is Rz(t) and a maximum height in thecircumferential direction is Rz(c), Rz(t)/Rz(c) is 1.2 or less.
 14. Themagnetic-disk substrate according to claim 9, wherein when a measurementpoint is provided every 30 degrees in the circumferential directionreferenced on the center of the substrate, and a radius of curvature ofa shape of a portion between the side wall surface and a chamferedsurface of the circular hole at the measurement point is derived, adifference in the radius of curvature between adjacent measurementpoints is 0.01 mm or less.
 15. A magnetic disk in which at least amagnetic layer is formed on a main surface of the magnetic-disksubstrate according to claim 9.